Power semiconductors are utilized in diverse applications for controlling current in accordance with a control signal such as a switching signal or a signal to be amplified in which one or more integrated circuits are contained within a hermetically sealed package. One or more control signal inputs are applied to the hermetically sealed package to produce a controlled high power current output signal. Such semiconductor hermetically sealed packages may be used for inverters for switching a pair of DC potentials applied to the hermetically sealed package under the control of a switching signal having a frequency of 400 Hz. for generating three phase alternating current in airframes at 400 Hz. The assignee of the present invention manufactures inverters which are used in airframes for switching DC potentials applied to hermetically sealed packages containing power semiconductor integrated circuits for producing three phase alternating current.
A first type of hermetically sealed power semiconductor integrated circuit package which has been developed by the assignee utilizes compression bonding. With compression bonding, a compressive force is applied to power semiconductor integrated circuits located within the hermetically sealed package to complete the necessary high power electrical connections for controlling the flow of high power current through the package. Compression bonded circuits for controlling the switching of large currents to produce three phase 400 Hz. alternating current have been developed by the assignee. For example, see U.S. Pat. Nos. 4,830,979, 4,954,876, 4,985,752 and 5,034,803. The present invention does not relate to this type of hermetically sealed power semiconductor integrated circuit package. Compression bonding cannot be used with insulated gate bipolar transistor (IGBT) power semiconductors.
A second type of hermetically sealed power semiconductor integrated circuit package, which does not utilize compression bonding to complete connections through which high power current flows to one or more semiconductor integrated circuits within the package, utilizes wire bonds which are connected to a plurality of electrodes on a first face of at least one semiconductor integrated circuit to a metallic bus. The individual electrodes are connected to circuits contained within the integrated circuit which are operated in parallel to provide a high power output by connecting the electrodes in parallel. One or more conductors, which are electrically connected to the bus, extend from inside of the hermetically sealed package to outside the package. A second face of the semiconductor integrated circuit functions as a second electrode which is in common with a plurality of individual circuits located within each semiconductor integrated circuit which function to amplify or switch current in parallel to provide the high power controlled current for use external to the hermetically sealed package. A metallic thermally conductive base is thermally connected to the second face of the semiconductor integrated circuit to provide a heat sink to dissipate heat caused by the operation of the circuits within the semiconductor integrated circuit. Moreover, the conductive base may function as one of the power terminals external to the hermetically sealed package.
While the use of one or more semiconductor integrated circuits contained within a hermetically sealed package which individually contain a plurality of individual circuits which produce a common output in parallel using wire bonds to produce a controlled current such as a switched or amplified high power current has diverse applications, there are a number of disadvantages consequent from the wire bonding between the power electrodes of the at least one integrated circuit and the external connections to the hermetically sealed package. First, the wire bonds represent an inductive load which slows the rate at which current may be changed in response to a control signal applied to the hermetically sealed package. Second, once the package is hermetically sealed, it is impossible to determine whether individual circuits within a semiconductor integrated circuit which are being operated in parallel to produce a high power output are, in fact, operating or operating up to power specification since there is no mechanism available to monitor the signal output or temperature of individual electrodes of the individual parallel circuits within a semiconductor integrated circuit.
U.S. Pat. No. 4,646,129 discloses a hermetically sealed package for semiconductor circuits. The '129 patent discloses electrical connections between a power semiconductor circuit located within a cavity of the hermetically sealed package and an electrical conductor mounted on an external surface of the package by conductors extending through a dielectric plate.
Recently, IGBTs have become available which have a highly desirable characteristic for switching high currents at high voltages to generate alternating current. However, IGBTs are not available in compression bonded configurations which are utilized with high power bipolar transistor switches. The advantage of IGBT is that smaller signal levels may be used for controlling the switching of a IGBT than a bipolar transistor.
Prior art electrical connections from inside of a chamber of a hermetically sealed power semiconductor integrated circuit package to the outside which are used to make electrical connections external to the power semiconductor integrated circuit within the package have used pins. The pins are typically brazed or sealed with glass or ceramic into an aperture through an insulator in a sidewall. The resistance of the pins for high power circuits may result in undesired heating and electrical loss and further contribute to the inductance of the circuitry within the hermetically sealed package which is undesirable for switching applications, such as those used in inverters.
External bus connections in the prior art hermetically sealed packages have been connected to pins extending through the walls or lid of the hermetically sealed package which produces an offset between the outer surface of the hermetically sealed package and the bus. This offset produces a higher profile package which increases the overall volume of the package which is undesirable in applications for inverters used for generating three phase alternating current in airframes where spatial utilization is important.
The breakdown voltage for a fixed gap of atmospheric air varies as a function of altitude in accordance with Paschen' Law which is expressed as: EQU V=f(.rho.d)
where .rho. is the gas density and d is the gap. As a result, a greater atmospheric spacing is needed at high altitudes of aircraft operation to prevent electrical breakdown between different electrical potentials of an electrical power generating system. As a result of Paschen's law, greater separation is required between different external buses of an inverter airplane electrical power generating system operated at different potentials at high altitude when compared to lower altitude operation. The geometry and materials of a hermetically sealed package containing the switches of an inverter electrical power generating system determine the available spacing between different potential buses and the package and mounting structure.